For hearing aids especially within the power category the above weaknesses presents a major problem as the high sound levels produced by these hearing aids result in high current consumption from the battery. If the maximum current consumption from the hearing aid is being limited to a conservative magnitude in order to avoid problems with battery voltage drop, the maximum sound pressure level that the hearing aid can supply will be clearly below the needs for some of the hearing impaired. Increasing the maximum current on the other hand will lead to another problem: if the battery voltage is below a certain limit, the settings of the hearing aid can not be guarantied and this results in a reset of the digital control circuit during which the hearing aid is inoperative. In other words some type of control within the hearing aid is needed that will allow us to take advantage of the high current which zinc-air batteries can deliver, but on the other hand restricts the current demand from the hearing aid if the battery voltage starts to drop.
A prior art system is described in published WO application No. DK03/00636. The prior art battery management system (BMS) uses a voltage detector to monitor the battery voltage as shown in FIG. 1. If this voltage goes down to a predefined voltage (called bat. Low or Vreference), the detector forwards a control signal Vcontrol. This signal is used to mute the output to the receiver which again will reduce the current drain from the battery thereby bringing the battery voltage up again combined with an un-mute of the signal. This scheme is repeated as many times as necessary in order to prevent the battery voltage from going down to the reset voltage of the hearing aid.
In the described prior art battery management system, advantage is taken of the class D amplifier used in the system. For a digital hearing aid with another type of output amplifier like a PCM amplifier this prior art scheme will not work due to audible side effects. According to the invention a system for battery management is provided, which can be used with a PCM amplifier without side effects detrimental to the sound quality.